The invention relates to a method for reducing chatter vibrations of a friction clutch controlled automatically by a clutch actuator on the basis of a target clutch torque assigned to a clutch torque which is to be transmitted, which clutch is positioned in a drivetrain of a motor vehicle between an internal combustion engine and a transmission, having a present actual clutch torque which is marked by vibrations as a result of chatter vibrations which occur occasionally. Automated friction clutches in a drivetrain of a motor vehicle between an internal combustion engine and a transmission have long been known. Here, instead of the foot of a driver, a clutch actuator controlled by a clutch device moves an actuating element, for example a clutch lever, a diaphragm spring, a lever spring or the like, along an actuation path. Assigned to the actuation path is a torque characteristic curve which is adaptable, for example, to external conditions such as clutch temperature, the frictional properties of the clutch linings, operation time and the like, and which may be calibrated by means of a contact point on the actuation path. For example, depending on the operating situation of the friction clutch desired by the driver or resulting from a transmission controller, a target clutch torque or a value connected therewith is ascertained by a control unit and issued as a control variable to adjust an actuation path of the clutch actuator corresponding to the target clutch torque. Depending on the design of the clutch actuator, when the clutch actuator is driven electrically this variable may be an electrical variable such as voltage, current or pulse width of a supply voltage, or a pressure, a volume flow or the like in the case of a hydraulically or pneumatically operated clutch actuator. The adjustment of the actuation path may be monitored or regulated by means of relative and/or absolute distance sensors.
In friction clutches of this sort, due to the geometric properties and manufacturing tolerances not corresponding to the ideal state, for example angle and axis offsets between the friction elements of the friction clutch resulting in uneven frictional engagement, so-called chatter vibrations may occur, in which case a disturbance clutch torque having a predetermined amplitude and frequency is superimposed on the clutch torque adjusted on the basis of the predetermined target clutch torque, which may result in disruptions of comfort of the motor vehicle and increased wear. To reduce such chatter vibrations, a method is known for example from DE 10 2012 217 132 A1 in which the frequency, amplitude and phase of a vibration superimposed on the transmission input signal is ascertained. In this method, depending on the frequency ascertained, an amplified and phase-shifted signal of the same frequency is generated and modulated as a control signal onto the target clutch torque, so as to eliminate oscillations of the transmission input signal. When multiple frequency components occur with comparable amplitude in the ascertained range, this compensation can result in behavior that is difficult to track. If, in addition, a phase jump occurs in the transmission input signal, frequency is difficult to determine; the same is true in the case of greatly changing amplitude or frequency, since modulations of amplitude, phase and frequency are interconnected.
In DE 10 2013 204 698 A1, a method is disclosed for damping drivetrain vibrations in a general form by eliminating a resonant frequency. This method provides only a limited possibility of reducing the chatter vibrations due to geometry. It is true that known excitation frequencies can be varied by adjusting filter parameters in order to perform an optimization for this excitation, but other excitation frequencies can be intensified thereby; for example, a chattering caused by geometry in a different frequency may be promoted.
From unpublished German patent application no. 10 2013 206 446.2 a method is known for reducing chatter vibrations of a friction clutch positioned in a drivetrain of a motor vehicle between an internal combustion engine and a transmission, which is controlled automatically by a clutch actuator on the basis of target clutch torque assigned to a clutch torque that is to be transmitted. In this case, chatter vibrations overlying a present actual clutch torque are corrected by ascertaining from a transmission behavior of the present actual clutch torque an amplitude and a phase of an input signal obtained at the output of the friction clutch and conveyed to a regulator, ascertaining from these parameters a phase-selective disturbance torque, ascertaining from the latter a phase-correct correction torque, and with this correcting the target clutch torque by means of a regulator which regulates the present actual clutch torque using the correction torque.